Old friend gets a stone (4)
I believe the answer is:
opal
'a stone' is the definition.
(gemstone)
'old friend' is the wordplay.
'old' becomes 'o' (abbreviation).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'gets' is the link.
(Other definitions for opal that I've seen before include "precious jewel" , "Type of gemstone" , "little jewel" , "Many-coloured gem" , "Milky-white gemstone with flashes of colour" .)